The characterization of the stiffness of materials is of importance in many fields. In particular, stiffness measurements can provide information on the physical states of materials that can be utilized to design objects, infrastructures, vehicles, etc. . . . . Stiffness measurements refer to a broad range of physical parameters measurements that include rheological properties, viscoelastic properties, shear stress and the like. Each of these subfields is governed by theoretical models that are used to extract specific values broadly related to the degree of rigidity of materials. The methods used to measure these properties are generally well known.
Certain types of soft materials are important economically and for public health and measurements of viscoelastic properties of these products provide valuable information. Two categories of such products are food products and biological materials.
Blood is one example of biological materials that is amenable to viscoelastic properties measurements. The mechanical properties of blood clots during the coagulation kinetic are related to the physiological state of the blood. There is an interest in characterizing blood coagulation in surgery to evaluate the coagulability of blood and prevent patient bleeding. In anesthesiology, the coagulation kinetic of blood is used to plan medication. The blood coagulation kinetic is also used as a point of care, emergency and laboratory tool to help diagnose blood disorders, which are related to many diseases, and to plan medication, to test anti-coagulant and procoagulant medicines.
The coagulation of blood is a complex polymerization process during which the blood changes from liquid to solid (coagulated) state. During clotting, the mechanical properties of blood change significantly over time due to the formation of a dense fibrin network in which are entrapped the red blood cells and other blood constituents. Since the change in mechanical properties is directly related to the composition and physiological state of the blood, several technologies and methods have been proposed to mechanically characterize blood coagulation kinetic. We can cite, among these technologies, the thromboelastography and the thromboelastometry. These techniques are inspired from rheometry. The blood sample is mixed with a coagulation reagent and poured in a cylindrical cup. A pin is inserted into the blood and the clot is formed between the pin and the cup. Depending on the technology, the cup or the pin oscillates at given frequency and amplitude to deform the clot. A probe measures the clot deformability under the external stress. The displacement of the moving part is recorded and qualitative data, related to the stiffness of the clot, are displayed as function of time. Thromboelastography and thromboelastometry are well established technologies that proved that mechanical characterization of blood during clotting provides relevant clinical information. However, these technologies are qualitative since they do not measure directly the elastic properties of the clot. Indeed, these technologies measure and display data that are indirectly related to the stiffness of the clot (in term of displacement, for example). Furthermore these techniques have relatively poor sensitivity and reproducibility.
The development of biomaterials for tissue regeneration and in vitro cell culture techniques are reshaping the future of medicine. The development of synthetic and biological biomaterials for medical uses implies rigorous investigations in: biology, physiology, pharmacology and mechanics. This latter is a key element in the success of in vitro and in vivo cellular culture, tissue regeneration and tissue engineering. Mechanical properties play an important role in the physiological functionality of tissues. For example, a vessel replacement tissue has to present precise viscoelastic properties in order to mimic (or reproduce) natural vessel tissue deformability, which critically impacts the regulation of the blood volume and pressure. Similarly, skin tissues engineering aims to produce flexible tissues in order to mimic skin flexibility and elasticity. It is then necessary to measure the viscoelastic properties of biomaterials in order to control their mechanical properties.
Among the technologies used to characterize in vitro biomaterials and cell cultures we can cite: rheometry, extensiometry and compression, indentation and atomic force microscopy (AFM). Rheometry and tension/compression present the disadvantage of destroying the sample, so it is very difficult to reuse the samples for multiple measurements over time. This important limitation in tissue engineering is due to the cost of the raw material, and the fact that cell growth can differ from one sample to another. Indentation and AFM do not damage the samples however, since measurements are very localized (micro and nano-scale for AFM), the viscoelastic characteristics are not representative of the bulk biomaterial properties. Furthermore it is difficult or impossible to use these techniques while keeping the sample sterile.
The food industry also benefits from measurements of viscoelastic properties in particular products related to milk and milk derivatives. Milk coagulates, under the action of special enzymes and coagulant agents, to form a soft gel. The coagulation and fermentation of animal milk is an important step in the preparation of food products like cheese, yogurt and other milk based soft solid products. The coagulation is also a key step in the preparation of tofu from soymilk. Coagulation of animal and vegetal milks has been extensively studied in food industry in order to formulate products, to design and set industrial production process and to control the quality of products. One of the challenges is to standardize the formation of gels. In the cheese industry, for example, it is critical to precisely select the cutting time in order to obtain a final product with the desired specifications (e.g. humidity) and to optimize the yield. The cutting time is directly related to the viscoelastic properties of the curd (the gel formed by coagulated milk). Depending on the variety of cheese, the suitable cutting time is the moment when the curd presents certain elasticity during the gelation kinetic. Cutting time has an important impact on the productivity in the cheese industry (amount of cheese per liter of milk). The preparation of yogurt also involves a viscoelastic change during the gelation kinetic (i.e. the fermentation of the milk). Depending on the final yogurt product, manufacturers will stop the fermentation process in order to produce a firm, stirred or liquid yogurt. Manufacturers have to take into account the viscoelastic properties of the yogurt in their product formulation and production process to reach a specific and constant quality and to improve productivity.
Consequently, there is a need to measure the viscoelastic properties of animal and vegetal milk during the coagulation kinetic. In research and development laboratories, instruments like rheometers are used to characterize the viscoelasticity of milk gels. However, these instruments are not adapted to test the quality of products (both in laboratory and at line) in production environments. Production plants generally use indirect methods to evaluate the viscoelasticity of milk gels. These methods can consist in the measurement, as function of time, of the acidity (pH) or optical properties (light backscatter) of the product. It can also consist in the use of penetrometers that indirectly reflects the overall stiffness state of a product. These methods lack specificity, reproducibility and precision. In some cases, the human expertise is the only one able to evaluate the stiffness of soft gels in plants (case of cheese manufacturers).
In view of the above there is a need for improved, more efficient and more precise methods and tools to measure the viscoelasticity of materials and especially soft materials.